Asbestos diseases: lung cancer

Advisory report | 26-07-2005

Causes of lung cancer

Each year in the Netherlands, 80 out of every 100,000 men and 27 out of every 100,000 women develop lung cancer. There were 8,800 new cases of lung cancer in 2000. Of these, 80 to 90 percent were caused by smoking cigarettes. A little over 10 percent of the cases among men are estimated to be attributable to exposure to asbestos at work.
The question of whether a malignant lung tumour is attributable to exposure to asbestos is much less easy to answer than the corresponding question in the case of a malignant mesothelioma. A mesothelioma is practically always caused by inhaled asbestos fibres. Therefore, if it is possible to make the diagnosis ‘malignant mesothelioma’ and to establish that the person concerned has been in contact with asbestos at work, then a causal link between exposure and the disease may be assumed to exist. This is not so in the case of lung cancer. Lung cancer is caused by various factors, of which cigarette smoking is the main factor at the population level. Very many people who have worked with asbestos have smoked, so the question of whether a malignant lung tumour should be attributed to smoking, asbestos exposure or some other malefactor can never be answered with a simple yes or no. This is because it is not clear from the tumour how it was caused.

Asbestos and lung cancer

Before further examining this question, the Committee first describes the way in which the diagnosis is made. As with the malignant mesothelioma, this is Step 1 in cases involving claims for damages. If the diagnosis lung cancer can be made with sufficient certainty, then the next question arises: Could the malignant lung tumour have been caused by inhaling asbestos fibres at work? This question can be subdivided. Firstly, does asbestos cause lung cancer? The answer is yes. It can be concluded with scientific certainty on the basis of extensive epidemiological and toxicological research that exposure to asbestos increases the risk of developing lung cancer. However, not all asbestos fibres have the same impact: long, straight fibres present a greater hazard than short curled fibres. Besides the shape, the chemical nature of the fibres is also important; blue and brown asbestos fibres present a greater hazard than white ones. Estimates of the increased likelihood of developing cancer after a given exposure period to a given airborne fibre concentration therefore tend to vary rather widely. The reason for this variation is found not only in the nature of the fibres but also in uncertainties about the extent and duration of exposure experienced by employees who are the focus of the research into the risk of lung cancer.
The increase in the risk of developing lung cancer can be described as to be proportional to cumulative exposure—the product the concentration of fibres in the air and exposure time. It is not possible for this to give an absolute safe level for exposure to asbestos. Cumulative exposure is generally expressed in fibre year; 1 fibre year equates with exposure to 1 fibre per cubic centimetre of air for one employment year.
Depending on the way in which the asbestos is used, the increased risk of developing lung cancer, i.e. the risk coefficient, varies from 0.1 to 5 percent per fibre year of cumulative exposure. For the situation in the Netherlands, given the lack of precise data on the type of asbestos, a risk coefficient of 1 percent per fibre year is a scientifically accountable starting point for estimating the increased likelihood of developing lung cancer. Higher figures are appropriate in case of regular exposure to the more hazardous fibre types.
Besides the risk coefficient, putting a figure on the increased likelihood of developing a malignant lung tumour also requires information on cumulative exposure. It is often difficult to determine cumulative exposure in a specific case. Particularly in the case of exposure that occurred several decades ago, there is often a lack of measurement data and it is difficult to obtain accurate information on the working processes that were used. The Committee deems the role of industrial hygiene experts to be essential here. The Committee believes it would be advisable to adopt a systematic method of discounting uncertainties in exposure estimates as far as possible, and to a greater extent than this is done at present.

The impact of smoking

A recurring question — and one that the State Secretary has put to the Health Council — concerns the extent to which the habit of smoking is important in the occurrence of lung cancer among people who work with asbestos. Research has shown that the effects of smoking cigarettes and exposure to asbestos fibres reinforce each other in the development of lung cancer. Precisely how this occurs is not yet completely clear. Many research results can be described with a multiplicative model: the increased risk of developing lung cancer for someone who has smoked and has also inhaled asbestos fibres is the product of the increase that would apply in the case of only smoking and that in the case of only being exposed to asbestos. This finding leads to the conclusion that the increased risk of developing a malignant lung tumour that can be attributed to asbestos exposure does not depend on the amount that a person has smoked (but, obviously, the risk of developing lung cancer does).
The few studies conducted that also involved people who had never smoked show that this relatively simple picture needs to be refined. The risk coefficient for non-smokers appears to be a factor of 3 times higher than that for regular smokers. The Committee believes this should be taken into account in claims for damages and, therefore, that the increased risk of developing lung cancer for non-smokers — defined by the Committee as people who have not smoked since they were twenty — is up to three times higher than the figure for smokers subjected to the same level of exposure.

Assessment of claims for damages

Although there are problems with uncertainties, it is possible, using data on the cumulative exposure and applying the available risk coefficients, under reasonable assumptions to put a figure on the likelihood that a malignant lung tumour was caused by occupational exposure to asbestos. This gives rise to the question of the impact this information has on awarding claims for damages. Although not specifically requested by the State Secretary, the Committee has addressed this question. The Committee has observed that the ‘all or nothing’ approach is common at the international level. This approach is found in insurance schemes for occupational diseases, statutory regulations in some countries, and also in liability under civil law. The claim is awarded if the risk of damage (in this case lung cancer) exceeds a certain threshold. The threshold is often 50 percent; that is the lung cancer is just as likely to have been caused by exposure to asbestos as by some other factor. However, in insurance schemes, uncertainties are often discounted in favour of the affected employee — which actually constitutes a reduction in the benefit threshold.
In recent years in the Netherlands, it was decided in various asbestos cases involving liability under civil law to choose what is known as a proportional approach. In such cases, the claim is awarded for the extent to which the malignant tumour can be attributed to the exposure to asbestos. This approach results in a claim only being partially awarded, but also results in the awarding damages in cases of relatively low exposure. The latter is in contrast with the ‘all or nothing’ approach, in which the claim would probably not be accepted in such cases.

Improving the calculation of the probability of causation

Finally, the Committee puts forward a proposal for reducing the uncertainties in the likelihood that the malignant lung tumour was cause by asbestos exposure—the calculation of the probability of causation. A further analysis of data from current or past studies might be useful in this respect. In the Netherlands, a few large groups of cohorts have been involved in epidemiological research into the occurrence of various diseases, including cancer. The Committee recommends to use the data from these research projects to obtain greater insight into the risk coefficients that apply in the Netherlands and to obtain more accurate information on the way in which the habit of smoking affects the likelihood of the development of lung cancer among people exposed to asbestos.